Scientists have known for a long time that the cochlea - the ear's sound sensor and transducer - contains two tubes with distinct ionic fluids, resulting in a difference in electrical potential. In other words, a battery. (Actually it's more of an AC generator, since the voltage fluctuates with the differences in pressure created by sound waves.) Until recently, though, engineers were unable to exploit this 100mV source of electricity.

Guinea pig cochlea, courtesy of The CELL: An Image Library

Researchers at the Massachusetts Institute of Technology (MIT) have created an implantable chip that harvests this previously untapped source of energy and powers the chip and an attached low-power radio transmitter. The fluctuating voltage produced by the cochlea induces an alternating current in the implant's electrodes. This current is converted to DC by the on-chip circuitry and used to power the device.

Image: Patrick P. Mercier

Because the energy is already being used by the eardrum to generate electrical signals, only a small fraction of it can be siphoned off to power the chips. Even so, there was enough energy - about 1 nanowatt - to power the sensor chip and transmitter for about 5 hours without disrupting normal hearing functions. (The device was tested on a Guinea pig. Human testing is planned for the future.) The project was led by Konstantina Stankovic, otologic surgeon at the Massachusetts Eye and Ear Infirmary. MIT’s Microsystems Technology Laboratories developed the low-power chip and transmitter.

Image: Patrick P. Mercier

While this is just a proof of concept for now, the researchers foresee a generation of implantable sensors that measure biological functions and transmit data to external readers, all being powered by the body itself. Immediate applications include measurements about the ear's functions, which will help medical researchers determine the causes of hearing afflictions.